The apparatus includes: an oscillatable unit; a driver/receiver unit, which has at least one signal input and one signal output, and which excites the oscillatable unit to oscillate, and/or which receives its oscillations; a control/evaluation unit, which has at least one signal input and one signal output, which controls the driver/receiver unit, and/or which evaluates the oscillations of the oscillatable unit; and at least one transmitting line and one receiving line between the control/evaluation unit and the driver/receiver unit; with the transmitting line being connected with the signal output of the control/evaluation unit and the signal input of the driver/receiver unit; and with the receiving line being connected with the signal output of the driver/receiver unit and the signal input of the control/evaluation unit. The process variables can be e.g. fill level, viscosity, density, pressure, temperature or the pH of the medium.
Measuring devices for the measuring and/or monitoring of the fill level of a medium in a container are available from the assignee under the mark “Liquiphant”. The principle of construction explained in the following for such measuring devices is, however, also true for similar measuring devices. Such a measuring device is usually composed of an oscillatable unit, a driver/receiver unit and a control/evaluation unit. The driver/receiver unit excites the oscillatable unit—in the case of the Liquiphant device, this is a tuning fork—to oscillate and receives the oscillations of the oscillatable unit. The frequency of the oscillations depends e.g. on whether the oscillatable unit is oscillating in air or, instead, is covered by medium. On this basis, the degree of covering can be deduced from the frequency. The same can likewise be derived from the amplitude; usually, however, the frequency is evaluated. In the driver/receiver unit, for example at least one piezoelectric element can be present, which converts an electric signal into a mechanical oscillation, which is then transmitted via a suitable membrane to the oscillatable unit. Corresponding considerations hold for the conversion of the mechanical oscillation into an electrical signal. A feedback electronics, which, in turn, amplifies and feeds back the signal of the oscillatable unit, and the electronics for evaluating the oscillation are combined in a control/evaluation unit. This control/evaluation unit is usually farther removed from the actual process, in order e.g. better to protect the electronics against the influences of the medium, e.g. high temperature. Usually located between the driver/receiver unit and the control/evaluation unit are at least two electric lines: a transmitting line and a receiving line. Mostly, there is also a ground cable, which likewise is extended to the driver/receiver unit. On the lines, the exciting signal reaches the driver/receiver unit, and the received signal reaches the control/evaluation unit. Depending on the requirements of the container or the plant to be monitored, these lines can be up to several meters long. In most cases, some type of tube surrounds these usually insulated lines, for their further protection. Therefore, the section between driver/receiver unit and control/evaluation unit is also mostly designated to be a tube extension. Fundamentally, despite all measures for sealing the interior of the tube extension, a penetration of moisture or other gases can not be completely eliminated. Due to temperature influences, it is also possible to experience outgasings of the internal parts or e.g. the adhesive. By condensation of the gases or moisture, an electrically conductive connection can then arise, in the form of a condensate bridge between the transmitting and receiving lines, via which the transmitted signal can affect the received signal. This can even lead to total failure of the measuring device. Such condensate bridges can be especially damaging to the plug connections needed for technical, manufacturing reasons near the electronics of the control/evaluation unit and, on the other end, near the driver/receiver unit. These plug connections can not be potted, such as is done e.g. in the case of the driver/receiver unit itself. A problem of a condensate bridge between transmitting and receiving lines is that the transmitted signal can leak, or crosstalk, onto the receiving line and, therefore, can disturb the received signal.